This invention relates to a rotary valve in particular for use in pulse tube refrigerators.
A typical use for a pulse tube refrigerator (PTR) is to maintain a cold environment in a thermal envelope surrounding a liquid helium cooled super-conducting magnet. Gaseous helium is generally used as the working fluid in the PTR. The operation of pulse tube refrigeration is well known.
In one form of pulse tube refrigeration, it is necessary for the helium gas supplied from a compressor to be fed in sequencexe2x80x94with a precisely controlled and repetitive cyclexe2x80x94to a number of interconnected chambers which comprise the PTR. The cycle rate, mark space ratio and phasing of these flow pulses are critical in achieving the required thermal performance. Discrete restrictors are also employed to separately control the inflow and the outflow of at least onexe2x80x94and typically twoxe2x80x94of the flow paths from the PTR chambers.
The flow switching described above can be achieved in a variety of ways, including banks of solenoid operated valves operated under electronic control. An alternative means is by a multi-port rotary valve device, driven at constant speed, such as by a synchronous or stepper electric motor drive. A well-known form of such a valve is the port plate rotary valve. A rotor, which turns against a port plate, incorporates flow control ports on its running face, which interact with matching port profiles on the port plate to provide the valving action. The flow logic and timing is therefore dictated by the geometry and the valving action of the flow commutation elements of the rotary valve. The rotor and the port plate ports are variously interconnected with the PTR chambers and the compressor supply and return. The valve rotor and drive motor may conveniently be contained within a single pressure vessel incorporated within the PTR flow circuit.
In one form of such a valve, a closure force between the rotor and the port plate is obtained by virtue of the differential pressure acting between the compressor supply and the compressor return. This pressure is arranged to act to directly load the valve rotor against the port plate by directing the gas supply pressure to the chamber enclosing the rotor. Additional provision is made to hold the rotor against the stator using a spring to ensure an initial seal is developed under start up conditions. This initial seal allows the differential pressure to be developed across the rotor as the compressor pressure builds, forcing the rotor against the running face of the port plate. The magnitude of the closure force is a function of the differential pressure, the rotor face geometry and the opposing pressures developed in the port plate ports.
This approach is effective for simple two port rotary valves. However, for multiple port arrangements, it limits the complexity of the rotor and port plate porting or alternatively results in an excessively heavy closure force, which is typically significantly greater than that required to achieve an effective seal at the rotor interface. This results in unnecessary rotor and port plate wear and excessive drive torque requirements.
In accordance with a first aspect of the present invention, a multiple port rotary valve for flow switching comprises a rotor, a stator, a compressor supply and a compressor return; wherein gas is supplied via the compressor supply throughout a switching cycle such that the rotor is lifted away from the stator; the valve further comprising a balance ram to counteract the effect of the differential pressure produced by the compressor supply, such that a gas seal is formed at a rotor/stator interface.
In the rotary valve of the present invention, the supply pressure approaches the rotor from the stator contrary to conventional rotary valves. The differential pressure across the rotor tends to lift the rotor off the stator face and this tendency is counteracted by the balance ram which is accessed by the same supply pressure. By enabling a sufficient, but not excessive closure force, wear of the rotor and stator is reduced.
Preferably, the balance ram is powered by the compressor supply.
Preferably, the valve further comprises bias means, such that a seal is provided at the rotor/stator interface at start up.
In use, axial force generated by the balance ram is carried by a thrust bearing arrangement. This may be a rolling element thrust bearing arrangement, but preferably the valve further comprises an axial hydrostatic bearing.
A helium gas environment as encountered in a PTR is known to leach lubricant from conventional bearings which may cause premature bearing failure and the leached lubricant may contaminate the PTR. The same supply pressure used to generate the balance force can be utilised in an axial hydrostatic bearing, with the prospect of virtually zero friction and a virtually indefinite life without the need for lubricant.
Preferably, the bearing comprises a floating disc having a relieved centre, such that gas trapped under the disc acts substantially equally over the area under the rotor to react an applied thrust force.
Preferably, the disc has an area greater than the cross sectional area of a piston of the balance ram.
Preferably, the rotor is coupled to a motor shaft with two degrees of angular freedom.
Preferably, the thrust bearing is mounted directly behind the balance ram.
In accordance with a second aspect of the present invention a pulse tube refrigerator (PTR) comprises a multiple port rotary valve according to the first aspect and a plurality of chambers to receive gas from the compressor supply.